The present invention relates to a sheet handling device, comprising a sheet support element, said element having a surface comprising suction holes which are connected to at least one suction chamber, said suction chamber being connected to a suction device adapted to create a subatmospheric pressure in the suction chamber.
In the copying and printing industry, a sheet support element with suction holes is frequently used for supporting an image receiving sheet and at the same time ensuring that the sheet lies perfectly flat on the support element. For example, in an ink jet printer, a sheet, e.g., a sheet of paper, is advanced over a sheet support plate while the image is being printed. The sheet is held on the sheet support plate due to the subatmospheric pressure in the suction chamber which is connected to suction holes facing the bottom side of the sheet. A certain subatmospheric pressure is required to hold the sheet sufficiently flat on the support plate.
When sheets of different width are to be printed, the smaller sheets do not cover the sheet support element completely. Therefore, some of the suction holes are not covered. When the suction chamber is made of one large compartment extending over the whole area of the sheet support element, an air flow through the not covered suction holes leads to an increase of the pressure in the suction chamber. This results in the sheet not being held firmly on the support element. If, on the other hand, a suction device of higher power is used to compensate for the increased air flow into the suction chamber, the suction will be too strong when all suction holes are covered by a large sheet, and the advance of the sheet over the support element is impeded.
From the European patent application EP 0 997 308 A2 a media hold down unit is known that comprises two or more vacuum chambers, a first vacuum chamber being directly connected to a vacuum source, while the other vacuum chambers are each connected to the vacuum source via separate bypass channels. The sizes of the chambers correspond to different widths of sheets that are to be handled. If one of the chambers is not covered by a sheet, an air flow is generated from this chamber through the bypass channel. The air flow affects the uniformity of the pressure inside the first vacuum chamber. Since the bypass channel and the first vacuum chamber are connected at an entrance into a conduit leading to the vacuum source, the uniformity of the pressure provided at the first chamber is affected only to a reduced extent. However, only a limited number of different sheet widths can be handled. Furthermore, for each partition of the support plate an extra bypass channel is needed and requires extra space.